Mongol-Jin War

The Mongol-Jin War was fought from 1211 to 1234 when the Mongol Empire under Genghis Khan conquered the Great Jin dynasty of northern China. The conquest of Jin was completed with the fall of Kaifeng to the Mongols, which gave the Mongols control of northern China.

History
In 1209, the Mongol emperor Genghis Khan conquered the Western Xia dynasty of northwestern China, and the neighboring Great Jin dynasty of northern China ignored the Xia's calls for aid. However, by 1211, they were on high alert for a Mongol invasion and demanded Mongol submission, leading to war. In March of 1211, Genghis Khan summoned all Mongol chieftains to wage war against China. Jin mobilized around 800,000 untrained peasants and 150,000 highly-trained heavy cavalry to fight the Mongols, but they were spread along the Great Wall and garrisoned at separate fortresses. The 90,000-strong Mongol cavalry army bypassed the Great Wall and split into two armies; Genghis Khan's 60,000-strong force invaded Jin as his son Ogedei Khan led 30,000 troops to besiege Datong. Genghis Khan met the Jin army at Yehuling on the road to Beijing, and the Mongols waited for the Jin assault for months. While waiting, Genghis sent Jebe and Subedei to attack the Jin army from the rear at Wushao Fortress. The Jin army and their commander were annihilated, and Genghis Khan simultaneously defeated the Jin forces at the Battle of Yehuling. Genghis sent his general Subedei around the pass and attacked the Jin army from both sides; Jebe launched a feigned retreat and left behind loot, leading to the Jin leaving their fortifications and being ambushed by thousands of Mongol archers. Genghis Khan now began raiding the countryside before withdrawing for the winter. In 1212, the Mongols struck again and besieged Datong, where Genghis was wounded by an arrow; the city held out for the winter. In 1213, the Mongols began besieging several fortresses and cities and waged guerrilla warfare, massacring or inhabiting most of the residents of the captured cities, but sparing and recruiting the elite. Soon, all Jin lands north of the Yellow River were secured, and Genghis Khan laid siege to Beijing. Emperor Xuanzong of Jin agreed to peace in exchange for loot, men, horses, and his daughter, along with subjugation to the Mongol khan, and the Mongols left China and returned to Mongolia with their treasure. When news arrived that the Emperor moved his capital to Kaifeng in preparation for a counterattack, Genghis Khan besieged, sacked, and razed Beijing, which burned for a month as its people were massacred. With Beijing taken in 1215, Genghis Khan conquered northern China and began to focus west, distracting him from his campaigns in China. After years of minor battles, Ogedei Khan launched a new offensive against the Jin in 1230, and the Jin abandoned Chang'an and withdrew to Kaifeng. In 1233, Emperor Aizong of Jin abandoned Kaifeng, taking shelter in Caizhou. The Mongols went on to ally with the Song, who besieged Caizhong in 1234. Aizong committed suicide when the enemy breached the walls, and his successor Emperor Mo of Jin reigned for just a day before being slain in battle. Caizhou's fall left northern China in Mongol hands, and the Great Jin was vanquished.